Corrosion inhibition of copper-zinc alloys

ABSTRACT

THE CORROSION OF COPPER-ZINC ALLOYS IN WATER DUE TO THE DISSOLUTION OF ZINC IS INHIBITED BY PASSIVATING EXPOSED SURFACES OF THE ALLOY WITH MAGNESIUM.

United States Patent O1 3,697,317 Patented Oct. 10, 1972 3,697,317 CORROSION INTHBITION OF COPPER-ZINC ALLOYS Charles R. Schmitt, Oak Ridge, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed May 19, 1971, Ser. No. 145,034 Int. Cl. C23c 3/00 US. Cl. 117-130 R 3 Claims ABSTRACT OF THE DISCLOSURE The corrosion of copper-zinc alloys in water due to the dissolution of zinc is inhibited by passivating exposed surfaces of the alloy with magnesium.

The present invention relates generally to increasing the resistance of copper-zinc alloys to dezincification, and more particularly to the treating of copper-zinc articles for providing a corrosion inhibiting surface film thereon. This invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

Copper-zinc alloys containing about 60 to 90 weight percent copper and the remainder substantially zinc are used extensively in applications involving contact with water since such alloys exhibit better resistance to corrosion than most other metals. However, even these alloys undergo a form of corrosion when exposed to water in that zinc is dissolved from the alloy leaving a relatively porous mass of copper which possesses little mechanical strength. This dissolution of zinc or dezincification is believed to result when the alloy dissolves with the copper depositing'or replating on the surface of the alloy in the form of a porous mass. Such dezincification may occur over the exposed surface of the alloy in a somewhat uniform manner or in a relatively small area which can lead to significant and deleterious penetration of the alloy. The strength, workability and/or corrosion resistance of copper-zinc alloys can be somewhat enhanced by employing in the alloys small amounts of additives such as tin, aluminum, arsenic, antimony, phosphorus, iron, lead, silver, manganese, and silicon. Even with such additives dezincification is still present to some extent.

Accordingly, it is the primary aim or objective of the present invention to obviate or substantially minimize dezincification of copper-zinc alloys upon exposure to water. This goal is achieved by providing surfaces of the copper-zinc alloys subject to contact with water with a thin film of magnesium.

Other and further objects of the invention will be obvious upon an understanding of the illustrative method about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

The present invention relates to inhibiting dezincification of copper-zinc alloys with zinc and copper in the aforementioned ranges and with or Without one or more of the above-mentioned additives. The corrosion or dezincification of the copper-zinc alloys is inhibited by passivating surface portions of articles of such alloys that are subject to contact or exposure with water in any form such as dimeneralized or high-mineral-content solutions such as sea water. The passivating of the surface of copperzinc alloy articles is achieved by applying a film or coating of metallic magnesium. This coating operation is effected by contacting selected surface portions of the alloy article with an aqueous solution of distilled or demineralized water containing -30 p.p.m. by weight of dissolved metallic magnesium. This concentration of magfice nesium in the solution is sufficient to provide a monolayer of magnesium -200 A. thick in a period of about 24 hours when the solution is heated to a temperature in the range of about 70-90 C. Thicker magnesium layers may be provided on the article, if desired, by increasing the concentration of magnesium in this solution and/or by contacting the alloy for a longer duration. However, such thicker coatings do not appear to provide any advantages over thinner coatings except for use in extreme corrosive conditions such as encountered in sea water over a long period of time. The magnesium may be added to the water in the form of ribbons approximately fii-inch wide by 0.015-inch thick by 2 inches long, turnings, wire, powder in a size range of approximately 50 to 100 microns diameter, or in any other desired form. The presence of the magnesium layer on the alloy surface is confirmed by a spark source mass spectrography examination which showed that the surface of a copper-zinc alloy before and after passivation with magnisum containing 500 p.p.m. and 5200 p.p.m. by weight magnesium, respectively. The presence of a thin protective layer of magnesium on the magnesium-treated alloy surface is also confirmed by electron microprobe examination. The electron backscatter image of the magnesium-passivated surface after 734 hours in demineralized water at F. shows distinct and uniform zinc and magnesium characteristic X-ray images and a relatively smooth, non-corroded surface. In contrast, the electron backscatter image of an untreated alloy surface after an exposure of only 162 hours in demineralized water at 160 F. shows a rough, pitted surface and a zinc-depleted and highly variable zinc concentration.

The mechanism of corrosion inhibition of copper-zinc alloy by the addition of metallic magnesium to de-- mineralized water heated to a temperature in the above range was studied by spark source spectrographic and electron microprobe analysts. As shown in Table 1 below, the corrosion of alloy was effectively inhibited over continuous exposure to demineralized water heated at 160 F. during an exposure period of 2717 hours (113 days).

TAB LE I Dezlneiflcation of brass valve in demineralized water at 160 F.

Exposure time Parts per million by weight Days Zn Cu Mg At the end of the 2717-hour exposure of the copper-zinc alloy it was noticed that some white colloidal solids had formed in the test vessel. These solids were filtered, air dried and analyzed by X-ray diffraction. The major constituent of the solids was identified as with some amorphous material present. No other intermediate or minor diffraction patterns were observed.

On the other hand a copper-zinc alloy article placed in demineralized water at 160 F. without the magnesium passivated surface thereon suffered considerable dezincification after only 162 hours of exposure as shown in Table II below.

The pH of the water has a substantial effect on the rate of dezincification in that at a pH value of about 10 the copper and zinc dissolve at about the same rate but at a pH in the range of about 67 the zinc dissolves at a rate about 240 times greater than that of the copper. Accordingly, the use of the passivated surface in accordance with the present invention is of particular usefulness when copper-zinc articles are subiected to contact with water having a pH value: of about 6 to 7..

The corrosion resistance of an untreated and a magnesium-treated copper-zinc structure, i.e., a flow control valve, was evaluated in static exposure tests to a saline solution closely corresponding to sea water. The salinity and mineral content of this water provides a relatively severe corrosive condition as compared to demineralized or other water. However, after a 6-month exposure to'this synthetic sea water aty25 C. it was found that the untreated copper-zinc alloy valve showed considerably more corrosion than did the magnesium-coated valve. This observation was substantiated by the greater amount of green corrosion-product crystals formed on the .article surface and by the comparatively deeper overall surface pitting. A colloidal, light-green, corrosion-product precipitate formed in each of the two vessels of the synthetic sea water used for accelerated corrosion testing of the copper-zinc valves. The amount of dried precipitate in the water containing the magnesium-treated valve was only approximately one-third the amount in the sea water containing the untreated valve (i.e., 0.1 gm. compared to 0.3 gm.). Also spectrographic analyses showed that the green corrosion product from the untreated valve had a zinc content of greater than 10 percentcompared to less than the detectable limit of five percent zinc for the green corrosion product formed by the magnesium-treated valve. X-ray diffraction analyses of the green corrosion product from the untreated valve also showed the presence of a major zinc-rich constituent in the deposit,

3 (Cu, Zn) S -H O whereas the corrosion deposit from the magnesium-treated retarded on the magnesium-treated valve. Although both valves were not identical in size, the exposed surface areas of the untreated and magnesium-treated valves were in the ratio of 1.3 to 1.0, respectively. Some surface corrosion in the form of pitting was observed on the magnesium-treated valve after the 6-month immersion in the synthetic sea water, but it was considerably less in severity than was the pitting on they untreated valve.

Electron microprobe studies of several different areas on the replicated untreated and magnesium-treated valve surfaces disclosed that although the magnesium-treated brass valve had shown some pitting after the 6-monthexposure, the original surface machining markings were still visible, whereas these surface markings had all been removed by corrosion on the untreated brass valve.

It will be seen that the present invention provides an inexpensive and highly effective mechanism for inhibiting dezincification of copper-zinc alloys in aqueous solutions with the extent of dezincification inhibition being significantly greater than heretofore obtainable with the use of difierent alloying additions such as described above.

What is claimed is:

1. A method for inhibiting the deziucification of copper-zinc alloys in water, comprising contacting surface portions of a copper-zinc alloy article subject to contact with water with an aqueous solution containing suflicient metallic magnesium to provide a magnesium coating on said surface portions.

2. The method of claim 1, wherein the magnesium is in the solution in a concentration of about 10 to 30 parts per million by weight, the solution is heated to a temperature in the range of to C., and wherein the zinc alloy is maintained in said solution for a duration adequate to provide the coating with a thickness of at least about angstroms.

3. The method of claim 2, wherein the metallic magnesium is added to the solution in the form of wire, turnings, powder, or ribbons.

References Cited UNITED STATES PATENTS 2,369,813 2/ 1945 Wilkins 75157.5 3,030,308 4/1962 Agnew 25274 FOREIGN PATENTS 123,813 3/1947 Australia 75-157.5 1,411,272 8/ 1965 France.

OTHER REFERENCES Chem. Abstracts 68: 16052h, 1968 Chem. Abstracts 69: 38338d, 1968.

RALPH S. KENDALL, Primary Examiner U.S-. Cl. X.R. 

